3.x: Action cache clear, limit environmental variables

[akarnokd]

GitHub Actions don't support clearing the cache so the suggested workaround is to key it with a version number from a secret that can be updated without changing the yml.

In addition, secrets are now scoped to their run block which should prevent them from leaking across unrelated steps.

[codecov]

Codecov Report

Merging #7238 (61a5809) into 3.x (5bb119b) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7238      +/-   ##
============================================
- Coverage     99.55%   99.54%   -0.02%     
- Complexity     6757     6758       +1     
============================================
  Files           747      747              
  Lines         47391    47391              
  Branches       6382     6382              
============================================
- Hits          47181    47175       -6     
- Misses           96       99       +3     
- Partials        114      117       +3     

Impacted Files	Coverage Δ	Complexity Δ
...l/operators/observable/ObservableFlatMapMaybe.java	92.25% <0.00%> (-2.82%)	2.00% <0.00%> (ø%)
...eactivex/rxjava3/processors/BehaviorProcessor.java	97.48% <0.00%> (-2.52%)	50.00% <0.00%> (ø%)
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (-1.59%)	2.00% <0.00%> (ø%)
...a/io/reactivex/rxjava3/subjects/ReplaySubject.java	98.96% <0.00%> (-0.63%)	49.00% <0.00%> (ø%)
...a3/internal/operators/flowable/FlowableCreate.java	99.02% <0.00%> (-0.33%)	6.00% <0.00%> (ø%)
...ternal/operators/observable/ObservableFlatMap.java	97.87% <0.00%> (+0.35%)	3.00% <0.00%> (ø%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	97.40% <0.00%> (+0.51%)	2.00% <0.00%> (ø%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	98.25% <0.00%> (+0.58%)	2.00% <0.00%> (ø%)
...ternal/operators/completable/CompletableMerge.java	98.64% <0.00%> (+1.35%)	2.00% <0.00%> (ø%)
...va3/internal/operators/parallel/ParallelRunOn.java	100.00% <0.00%> (+1.46%)	8.00% <0.00%> (ø%)
... and 1 more

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